The present invention relates to a method of and apparatus for controlling the air-fuel ratio of an air-fuel mixture which is to be supplied to an internal combustion engine. More particularly, the invention is concerned with an air-fuel ratio controlling method and apparatus in which the mode of air-fuel ratio control is switched selectively in accordance with the state of the engine operation between a feedback control mode in which the air-fuel ratio is controlled in conformity with the stoichiometric one and a feed-forward control mode referred to as "lean control" in which the control is made to maintain an air-fuel ratio greater than the stoichiometric value, i.e. to maintain a mixture leaner than the stoichiometric one.
Generally, in automotive engines equipped with an exhaust scrubber using a ternary catalyst type, it is necessary to effect the air-fuel ratio control such that the air-fuel ratio, which is directly related to the condition of combustion in the enigne, is always maintained around the stoichiometric level, in order to keep the exhaust emissions clean.
To cope with this demand, a feedback control method has been proposed and used in which the oxygen content in the exhaust gases is detected by an O.sub.2 sensor as an index of the air-fuel ratio of the mixture, and the air-fuel ratio control is conducted in accordance with the output from the O.sub.2 sensor such that the air-fuel ratio coincides with the stoichiometric ratio.
When the engine is operating under comparatively light load, it is possible to decrease the rate of fuel consumption by maintaining the air-fuel ratio at the leaner side of the stoichiometric value without being accompanied by substantial degradation of the exhaust emissions because, under the light load, the rate of generation of nitrogen oxide is sufficiently small. Under these circumstances, an automotive engine has been proposed in which the control operation mode is selectively switched between the feedback control mode for maintaining the air-fuel ratio at the stoichiometric level and the lean control mode for maintaining the mixture at the leaner side of the stoichiometric one through a feed-forward control, thereby to keep the exhaust emissions clean while minimizing the fuel consumption.
In the system for controlling the air-fuel ratio, several conditions of the engine are detected to discriminate whether a condition for lean control mode is met or not, which include at least a cooling water temperature being higher than a reference temperature, an engine load being less than a reference load and vehicle speed being higher than a first reference speed. The fulfillment of the condition for lean control mode causes the engine to be operated under the lean air-fuel ratio. However, if the engine is required to be slightly accelerated while the condition for the lean control mode is fulfilled, the performance of the engine may not fully meet with the driver's desire due to the lean air-fuel ratio. Such an undesirable performance of the engine that the required slight acceleration is not effected has been experienced in the conventional lean control apparatus especially when a throttle valve being fully closed so as to cause the engine to be decelerated is gradually opened.
The selective use of the feedback control mode and the lean control mode, however, imposes the following problems particularly when the engine is used in heavy vehicles such as automobiles having weight of 1.25 tons or more or in vehicles having automatic transmissions.
When the vehicle, which is running at a constant speed of less than 25 Km/h for example, is to be accelerated gently, or when the vehicle is to be accelerated gently after deceleration with full closing of the throttle valve, the conditions for the lean control mode may be fully met. At this time, the lean control is executed, so that the driveability is impaired due to insufficient output torque of the engine.
In such a case, it would be possible to dismiss the lean control by kicking down the accelerator pedal, thereby to switch the control mode to the feedback control mode to obtain the large output torque. This, however, is not preferred because the kick down of the accelerator pedal causes an impractically large fuel consumption.
In some automotive engines, the conditions for execution or dismissal of the lean control include the degree of acceleration of the engine detected through calculation of the variance of the engine speed or vehicle speed at a period of one to several seconds, rather than through the detection of variance of the opening degree of the intake throttle valve or the variance of the intake pressure. In such engines, there is a fear that the air-fuel ratio control is made in the lean control mode for at least one to several seconds even when there is a demand for a acceleration including a comparatively quick acceleration during running at low speed. Consequently, the driver faces the same problem as described above.